This invention relates to a distributed backwards-wave balun for use, for example, in wireless, cellular handsets and radios, and in RF modules therefor.
Differential circuits have been employed in wireless cellular communications handsets and other wireless technologies for many years. The benefits from using differential circuits are lower noise and lower susceptibility to interference. Despite the benefits of differential circuits, some of the components used in a modern wireless communications technologies remain single ended; for example, single ended antennae are more common than differential antennae, and similarly it is often preferred to employ single ended power amplifiers. In cases where wireless communications technologies share single-ended and differential components, it is necessary to include devices which convert the unbalanced signals which are output from the single ended components to balanced signals which can be fed to the inputs of the differential components and vice versa.
Such devices are often referred to as baluns. A balun transforms a signal referenced to ground into two signals with equal amplitude and opposite phase. Figures of merit for describing the electrical characteristics of a balun are the amplitude and phase balance and the return loss and insertion loss.
A balun can be implemented by a number of discrete components. Balun topologies employing discrete components are described in U.S. Pat. Nos. 5,949,299 and 6,396,632. Baluns can also be implemented using distributed components; such baluns normally employ a number of half- or quarter-wavelength coupled transmission lines. A popular form of the distributed balun is described in N. Marchand: “Transmission Line Conversion Transformers”, Electronics, vol. 17, pp 142-145, 1944 and is often referred to as a Marchand balun after the inventor. An alternative distributed balun is described in U.S. Pat. No. 06,292,070 and is often referred to as a backwards-wave balun. The structure of a Marchand balun is depicted in FIG. 1a, and the structure of a backwards-wave balun is depicted in FIG. 1b. 
In each case the balun comprises first and second pairs of coupled transmission line sections 10A, 10B and 12A, 12B respectively. Each of the line sections 10A, 10B and 12A, 12B has an electrical length E which is equal to one quarter of the wavelength of the centre frequency of the operating band of the balun. The electrical characteristics of the coupled transmission line sections 10A, 10B and 12A, 12B are described by the electrical length E, by the even mode admittance YE and by the odd mode admittance YO of the coupled line sections. The line sections 10A and 12A are connected in series. In the case of the Marchand balun (FIG. 1a) the differential port 14 is connected across the inner ends of the line sections 10B, 12B and the single-ended port 16 is connected to one outer end of the series-connected line sections 10A, 12A. In the case of the backwards-wave balun (FIG. 1b) the differential port 14 is connected across the outer ends of the series-connected line sections 10A, 12A and the single-ended port 16 is connected to the inner end of one of the line sections 10B, 12B. Such baluns are so well-known that no further description is deemed necessary.
Distributed baluns such as the Marchand balun and the backwards-wave balun offer excellent performance in the areas of amplitude balance, phase balance, return loss and insertion loss; they also have a much wider bandwidth than the discrete balun described in U.S. Pat. No. 5,949,299.
Distributed baluns can easily be implemented in multilayer substrates using, for example, LTCC (low temperature co-fired ceramic) technology, and offer greater flexibility in the layout than baluns which employ discrete components, such as those described in U.S. Pat. Nos. 5,949,299 and 6,396,632. For example, a distributed balun can be fabricated in a multilayer LTCC substrate such that the coupled lines are folded over several layers of LTCC and where the metal patterns on each layer are connected to those on higher or lower layers by electrically conducting via holes. This structure can substantially reduce the XY dimensions of the balun if a sufficient number of layers of LTCC are used. On the other hand, the coupled lines can be confined to the surface of a single layer of LTCC, thereby substantially reducing the height of the balun at the expense of increased size in the XY plane. Distributed baluns can readily be matched to a range of input and output impedances without the need for matching components.
As described above, conventional Marchand and backwards-wave baluns comprise 2 quarter-wave coupled-line sections. At 2.45 GHz, the centre frequency for 802.11 b/g Wireless-LAN standards, a quarter-wave transmission line, fabricated on a substrate with a dielectric permittivity of 8 (typical for LTCC), will have a length of 11 mm. For mobile cellular applications, a balun employing a pair of 11 mm coupled line sections is rather large, and it is difficult to incorporate such long lines into a multilayer substrate with dimensions similar to those which are possible with the discrete balun described in U.S. Pat. No. 5,949,299. However, the wider bandwidth which distributed baluns can offer is increasingly becoming a requirement as cellular handsets and wireless technologies are designed to offer higher rates of data transfer and to operate on wider bands or on a greater diversity of bands. Clearly, there exists a strong demand for a balun which combines the wide bandwidth of the distributed balun described in U.S. Pat No. 06,292,070, together with the small size of the discrete balun described in U.S. Pat. No. 5,949,299.
Gavela I., Falagan M. A., Fluhr H.; “A small size LTCC balun for Wireless Applications”; Proceedings of the European Microwave Conference 2004; pp 373-376 showed that capacitive loading can offer substantial size reduction of a Marchand balun. Gavela et al found that by connecting capacitive loads to the unbalanced input and to the open circuit end of the series coupled line sections of a Marchand balun, a size reduction of ˜50% was possible.
U.S. Pat. No. 6,819,199 also discloses a compact Marchand balun. The size reduction of the balun of U.S. Pat. No. 6,819,199 is achieved through the use of multiple coupling or loading capacitors, as described on page 6, lines 42-51 of U.S. Pat. No. 6,819,199.
Marchand baluns have the drawback that the differential outputs are connected to ground via the grounded coupled lines. As a result, DC blocking capacitors are required if a DC bias is to be applied to the differential outputs of a Marchand balun—see FIG. 2a. A further drawback is that a pair of DC bias networks are required in order to apply a DC bias to both of the differential outputs—see also FIG. 2a. 
On the other hand, a DC bias can be applied to the differential outputs of a backwards-wave balun without the need for DC blocking capacitors, because the differential outputs of the balun are isolated from ground—see FIG. 2b. Furthermore, a DC bias can be applied to both differential outputs of a backwards-wave balun simultaneously by a single DC bias network—see also FIG. 2b. 